1. Technical Field
The disclosed embodiments relate in general to an apparatus for generating a pulse train, and more particularly to an apparatus for generating a pulse train with an adjustable time interval.
2. Description of the Related Art
In the prior art, a picosecond laser adopted in material micromachining comes in a picosecond laser with single pulse, as shown in FIG. 1A, or a picosecond laser with pulse train having an unadjustable time interval, as shown in FIG. 2A. Under a same amount of laser energy, compared to a drilling depth of the picosecond laser with single pulse shown in FIG. 1B, a picosecond laser with pulse train has a greater depth as shown in FIG. 2B. Referring to FIG. 1C, the picosecond laser with pulse train hence offers preferred surface processing effects. In the prior art, in addition to necessary signal synchronization and delay control, a conventional mechanism for generating the picosecond laser with pulse train is also complex in structure and high in cost as well as having an unadjustable pulse train time interval. FIG. 4 shows a state change of a material during a laser process, where the horizontal axis represents a material density and the vertical axis represents a material temperature. After being processed by a laser, the material enters a liquid phase from a solid phase, and then enters a gas phase. If the time interval of the pulse train is too long, a cutting amount is lowered when the material is cooled to below a critical point after the laser process, as shown in FIG. 5. In FIG. 5, the horizontal axis represents the time interval of the pulse train, and the vertical axis represents the cutting amount. As the cutting amount at the vertical axis decreases as the time interval at the horizontal axis increases, it is concluded that the time interval cannot be too long. Referring to FIG. 6, if the time interval of the pulse train is too short, plasma shielding effects are generated after the laser process. In FIG. 6, the horizontal axis represents the time. An area of plasma shielding generated due to an inadequate time interval blocks a next laser pulse when the material is still in the liquid phase after the laser process, signifying that the time interval of the pulse train cannot be too short, either. Further, time intervals of pulse trains for different materials may also be different. Therefore, the time interval is a critical processing parameter for laser pulse trains. In a conventional method for generating a picosecond laser with pulse train, from a high repetition rate laser pulse optical source, an electrically-controlled high-speed optical pulse picker selects a desired pulse train shape. However, such method limits the time interval between the pulse trains as the interval is unadjustable.